It has been a while since I said I would start working on Gaia's DR2 implementation in Space Engine (and I haven't show anything until now). A lot of decissions are been made and we are still probing how well behaved is the astrometry and the photometry of this second data release to try generate realistic estimates for the spectral types of each source and how far we can assume the inverse of the parallax to be a good estimator of the actual distance.

A lot of valid information is coming out at this point and I don't want you to wait indefinitely so I've implemented Gaia's DR2 White Dwarf catalog in SE. This is the first Gaia DR2 population to be reduced to a catalog since White Dwarfs are easily identifiable in Hertzsprung-Russell diagrams as a detached region.

304 DZ(Metals present in the atmosphere with atomic metal lines all over the optical spectrum)

286 DQ(Atomic and molecular Carbon lines are dominant in the optical spectrum)

66 DAZ(Hydrogen and Metal lines are visible in the optical spectrum)

34 DAB(Hydrogen and Helium I lines are both strong in the optical spectrum)

26 DBZ(Helium I and metal lines are visible in the optical spectrum)

5 DAO(Hydrogen and Helium II lines are both strong in the optical spectrum)

3 DX(Exotic spectrum with significant peculiarities)

1 DO(Helium II lines are dominant in the optical spectrum)

The temperatures used for Space Engine do not exactly coincide with those of the actual catalog because I randomized them using gaussians centered on the real measured values and standard deviation equal to the uncertainties in temperature. This was done to avoid unrealistic round values (instead of 7000 K it is more natural to use for example 7137 K, which is still consistent with the uncertainties).

How to install this addon?Download and save the "White_Dwarfs_GaiaDR2.csv" file inside your SpaceEngine/addons/catalogs/stars folder (create the necessary folders if they aren't there).

The size of the file is just 7.54 MB. If you see some catalog error please report it in this thread so I can fix it.

Why this catalog is so important?The astrometric limiting criteria used here is to have parallax errors below the 10% of the actual parallax values. In that way we can still assume the inverse of the parallax to be a good unbiased estimator of the distance. For larger errors several corrections will come before the third Gaia data release (DR3) which will account for Lutz-Kelker bias (and some artifacts in the data created by Gaia's scanning law). But for now 73.221 White Dwarfs with very accurate 3D positions are available in what is currently the largest White Dwarf catalogue ever made by far. As you can see in the video, the stars are concentrated in a spheroid of roughly 3.000 ly in diameter. Gaia can see a lot farther than that but the cut in 10% of relative error for the parallaxes and the fact that White Dwarfs have low luminosity (because of their small radius) limit the size of the sample to that volume of space.

Another important feat is that for the first 100 pc around the Solar System we have virtually reached completeness. This means that in the central ~650 ly diameter sphere we believe that those are all the White Dwarfs that actually exist in real life. If there is a White Dwarf inside this volume Gaia DR2 has probably detected it by now. For Space Engine this means that in future versions procedural generation for White Dwarfs in this region should be lowered to zero.

Another interesting thing is that we now have the most accurate density distribution value for White Dwarfs (that will tweek procedural generation parameters in the Milky Way); 4,9 White Dwarfs per cubic parsec (with a 0,4 uncertainty).

White Dwarfs can easily be separated from other stellar types by using accurate HR diagrams like the above. But for that we need good absolute magnitude values, which are calculated on apparent magnitude and distance. So it all depends on the accuracy of the parallax measurements in the end and that's why Gaia yields the best so far. There are probably many more White Dwarfs in DR2 waiting to be discovered but since their parallax measurements have higher than 10% errors it can be a tricky thing for now. In 2021 we will reach DR3 and with that many thousand more will be available.

Regarding this I talked previously with Vladimir and he think it is better to have this as an addon for the moment until we can integrate a significant amount of Gaia data in Space Engine. Why? Because in this way we avoid having unrealistic proportions between populations in the Solar neighbourhood (73.000 White Dwarfs is nearly the 40% of all the catalogued stars that would be in SE, which is absurd).Vladimir is thinking ways of implementing large datasets in the next version (after 0.9.9.0 I mean), and I'm working on bringing part of Gaia's DR2 catalog (around 60 to 70 million stars with good parallaxes) as addons. All of this could indeed be part of the official release of the version that comes after 0.9.9.0

Source of the post This means that in the central ~650 ly diameter sphere we believe that those are all the White Dwarfs that actually exist in real life. If there is a White Dwarf inside this volume Gaia DR2 has probably detected it by now.

But this is solitary white dwarfs? WD in binary systems, which was not resolved by GAIA, still lurking there. Taking the roughly 1:1 count of solitary : binary systems, we could assume that actual number of white dwarves in that radius is two times larger.How to compute average distance between dwarves? 4.9 pieces per cubic parsec seems looks very large amount, is it correct? I want to apply this data to the idea of gravity-assisted interstellar probes.

Source of the post But this is solitary white dwarfs? WD in binary systems, which was not resolved by GAIA, still lurking there.

Exactly! Yes, I should have noted that. If there is a high [math]\Delta m in a binary (like the case of Sirius), the high contrast would still hide the White Dwarf. We have reached completeness of single White Dwarfs.

That doesn't mean that Gaia has not resolved many White Dwarfs in binary systems. In fact one thing I wanted to work on in this thread is the addition of those. There are 120 binary systems containing White Dwarfs in the catalog that I've just added as a single star (using the spectral class of the primary). The next post I will add a table with their data so If anyone is willing to do a saparate file for the 120 binary systems it would be awesome (I don't think programing it would be faster with only 120 systems). There are White Dwarf - M-type star systems, White Dwarf - Brown Dwarf systems and even some White Dwarf - White Dwarf systems (as you can see all cases of low contrast in magnitude).

I had a conversation with the first author the other day and he told me that they estimate some 400.000 to 500.000 White Dwarfs to be discovered in Gaia's DR3 (the data release that is expected to arrive in 2021). So this is going to explode. Also the volume where we reach completeness will expand significantly. Gaia has probably detected the vast majority of those in DR2 but we need better parallax and photometry to confirm they are indeed WDs.

For the 120 White Dwarf binaries I ask for a community addon here. You can take the data from the attached file. The spectral classes are similar to those used in SE but there might be some differences.

For example, a DAB2.4+M is a binary with a DAB White Dwarf (SE allows for the specific kind of DAB star, so you can put DAB2.4 if you want in the catalog), with an M-type star as a companion. Another example, a DA+DA is a pair of White Dwarfs (both of the same DA class accepted by SE). One last example, sdB+F is a binary of a Blue sub-dwarf (in SE it can be included as a generic WD) with an F-type star as a companion.

Some things that might be complex to interpret (SE is much simpler so in case there is some doubt ask here or see the reference of the SVO columns or just add it as a generic star). For example a DO:+Ke system is composed of a DO White Dwarf and a K-type star but the "e" of the K star means that there are emission lines in the spectrum (not important for SE) which could mean the star has a close equatorial disk of material around, and the ":" of the DO star means there are "peculiarities" in the spectrum of it (also not important for SE) that might mean the star has been intoxicated with the material of a recently swallowed planet (take it as an example since spectral peculiarities could mean many many different things).

The DA/DC+M1.6-K2 is another interesting example; the "+" marks that it is a binary system (a DA/DC star with a M1.6-K2 star). The "/" of the DA/DC star means that it has spectral features of both DA and DC spectral types (In SE you have to specify so you should choose randomly between the two DA and DC classes). The "-" in the M1.6-K2 star means that the object might have changed spectral type during the time it has been observed, in this case a M-type star has changed to a K-type star; it could mean that the star is variable and transitions periodically from one temperature to another or it could mean that the star is evolving quickly after its departure from the main-sequence (only O-type and B-type stars evolve as quick as to be noticed in human timespans so don't worry because these are only late-type objects at best).

The "?" mark in some cases might mean that the spectral type of the companion it is not known or that even if it has a companion at all is truly confirmed (depending on where the sign has been placed). In general the question mark means there is high uncertainty in some spectral features. An example of a case where it is not known if it truly has a companion would be a DZ(+F?) and an example when it is knwon that there is a companion but there is uncertainty on the spectral type of it would be DZ+F?.

There might be spectral types not accepted by SE. For example a DBA or DXP White Dwarfs (the second is a DX White Dwarf with a strong magnetic field that polarizes light). In those cases I would add them as generic WD until SE develops a more complex spectral classification scheme or simplify it to the closest spectral type (for example the DXP could be a DX in SE without problem).

There could also be catalog errors (don't fear that since spectral types are difficult to determine with Gaia which was not specifically designed for that but for astrometry). For example there is a DB3+ star that I don't really know what means (probably a single star or maybe a binary where they forgot to add the companion).

The rest is easy. RA is in degrees so you have to put it in hours for the SE file. Use the distance column for distances in parsecs instead of computing it from the parallax column (even if the result is the same).

The temperature values should be a bit randomized since that would look unnatural (all rounded) in SE. And the temperature of a binary usually means the temperature of the primary (but in this case I think it is always the temperature of the White Dwarf of the system). The uncertainties in temperature of this sample vary across temperature ranges (I used them to have deviations from the temperature value compatible with he data):

From 5.000K to 20.000K the uncertainty is 250K

From 20.000K to 30.000K the uncertainty is 1.000K

From 30.000K to 40.000K the uncertainty is 2.000K

From 40.000K to 50.000K the uncertainty is 5.000K

From 50.000K to 80.000K the uncertainty is 10.000K

For example; if we have a star with a temperature of 9.000K, then a perfectly reasonable value for the SE file would be 8.923K, but a value of 9.325K would start to be way off considering the uncertainties involved in the measurement.

As I said, these are only 120 cases where we need to make a separate file but for the rest the 73.221 White Dwarfs of the catalog should remain untouched until 2021 (when we get the third data release from Gaia with new White Dwarfs and new data for the current ones).

Source of the post One last example, sdB+F is a binary of a Blue sub-dwarf (in SE it can be included as a generic WD) with an F-type star as a companion.

"sdB" is a valid spectral class in SE. It is equivalent of "B VI".I tend to think that spectral class in SE must be handles as string instead of encoded binary value. This allows to accept any kind of strange markings like : + ?. But strings are bad in that sense that they have indefinite length, but all 'star' elements in the array must be of the same size. For names (which are also strings with indefinite size) I solved that by putting all strings into a single chunk of memory and refer to them by a pointer from the star array element.But implementing string for the star class will virtually enable any kind of typos and errors in file catalogs. SE still need to determine the star type to display it in GUI (main sequence, giant, dwarf) and to calculate color for the particle (if it not provided in the catalog).

Ups, you are totally right. It wouldn't make sense if SE didn't have that sorry.And yes spectral classification is quite "an art" at some point. There are a lot of schemes when you go in detail and even differences between individual astronomers. So this would be messy in SE for sure (just looking Simbad one can get lost with the variety of the nomenclature).

For the 120 White Dwarf binaries I ask for a community addon here. You can take the data from the attached file. The spectral classes are similar to those used in SE but there might be some differences.

For example, a DAB2.4+M is a binary with a DAB White Dwarf (SE allows for the specific kind of DAB star, so you can put DAB2.4 if you want in the catalog), with an M-type star as a companion. Another example, a DA+DA is a pair of White Dwarfs (both of the same DA class accepted by SE). One last example, sdB+F is a binary of a Blue sub-dwarf (in SE it can be included as a generic WD) with an F-type star as a companion.

Some things that might be complex to interpret (SE is much simpler so in case there is some doubt ask here or see the reference of the SVO columns or just add it as a generic star). For example a DO:+Ke system is composed of a DO White Dwarf and a K-type star but the "e" of the K star means that there are emission lines in the spectrum (not important for SE) which could mean the star has a close equatorial disk of material around, and the ":" of the DO star means there are "peculiarities" in the spectrum of it (also not important for SE) that might mean the star has been intoxicated with the material of a recently swallowed planet (take it as an example since spectral peculiarities could mean many many different things).

The DA/DC+M1.6-K2 is another interesting example; the "+" marks that it is a binary system (a DA/DC star with a M1.6-K2 star). The "/" of the DA/DC star means that it has spectral features of both DA and DC spectral types (In SE you have to specify so you should choose randomly between the two DA and DC classes). The "-" in the M1.6-K2 star means that the object might have changed spectral type during the time it has been observed, in this case a M-type star has changed to a K-type star; it could mean that the star is variable and transitions periodically from one temperature to another or it could mean that the star is evolving quickly after its departure from the main-sequence (only O-type and B-type stars evolve as quick as to be noticed in human timespans so don't worry because these are only late-type objects at best).

The "?" mark in some cases might mean that the spectral type of the companion it is not known or that even if it has a companion at all is truly confirmed (depending on where the sign has been placed). In general the question mark means there is high uncertainty in some spectral features. An example of a case where it is not known if it truly has a companion would be a DZ(+F?) and an example when it is knwon that there is a companion but there is uncertainty on the spectral type of it would be DZ+F?.

There might be spectral types not accepted by SE. For example a DBA or DXP White Dwarfs (the second is a DX White Dwarf with a strong magnetic field that polarizes light). In those cases I would add them as generic WD until SE develops a more complex spectral classification scheme or simplify it to the closest spectral type (for example the DXP could be a DX in SE without problem).

There could also be catalog errors (don't fear that since spectral types are difficult to determine with Gaia which was not specifically designed for that but for astrometry). For example there is a DB3+ star that I don't really know what means (probably a single star or maybe a binary where they forgot to add the companion).

The rest is easy. RA is in degrees so you have to put it in hours for the SE file. Use the distance column for distances in parsecs instead of computing it from the parallax column (even if the result is the same).

The temperature values should be a bit randomized since that would look unnatural (all rounded) in SE. And the temperature of a binary usually means the temperature of the primary (but in this case I think it is always the temperature of the White Dwarf of the system). The uncertainties in temperature of this sample vary across temperature ranges (I used them to have deviations from the temperature value compatible with he data):

From 5.000K to 20.000K the uncertainty is 250K

From 20.000K to 30.000K the uncertainty is 1.000K

From 30.000K to 40.000K the uncertainty is 2.000K

From 40.000K to 50.000K the uncertainty is 5.000K

From 50.000K to 80.000K the uncertainty is 10.000K

For example; if we have a star with a temperature of 9.000K, then a perfectly reasonable value for the SE file would be 8.923K, but a value of 9.325K would start to be way off considering the uncertainties involved in the measurement.

As I said, these are only 120 cases where we need to make a separate file but for the rest the 73.221 White Dwarfs of the catalog should remain untouched until 2021 (when we get the third data release from Gaia with new White Dwarfs and new data for the current ones).

Should this be put in the same folder as the white dwarf add on was put into? Thanks!